Supress ticket update emails for "Machine" field

For the sake of making our inventory easy to manage, we regularly delete machines whose agents have not checked in for two weeks or longer.

When this is done however, all Help Desk tickets associated with a deleted machine are then updated to reflect the change (Machine changed from XXXXXXXX to Unassigned). This update then fires off a notification email to both the submitter and the ticket owner. Almost 99% of the time, the tickets in question have long been closed and this tends to generate confusion amongst both the submitters and the ticket owners.

Is there a way to prevent the ticket update emails from being sent when the only change was the value of the "Machine" field? Or perhaps there is a way to prevent notifications emails from being sent at all when the affected ticket is in a "Closed" state?

I would also like to clearly state that this is NOT in regard to deleted machines causing tickets to reopen. That issue has already been resolved for our usage. I simply need a way to prevent the ticket updated emails from being sent in regard to "Machine" field value changes or a way to prevent update emails from being sent in regard to tickets that are already closed.